Ferromagnetic thin metallic film magnetic recording media in which a ferromagnetic thin metallic film is formed on a non-magnetic support, by a physical deposition method such as vacuum deposition and sputtering or by a plating method, are known. Examples thereof include magnetic tape on which Co is deposited (Japanese Laid-Open Patent Publication No. 147010/1979) and a perpendicular magnetic recording medium in which Co-Cr alloy is used (Japanese Laid-Open Patent Publication No. 134706/1977). The thin metallic film formed by such film-forming means as vacuum evaporation, sputtering or ion-plating is as extremely thin as no more than 1.5 .mu.m. Nevertheless, it has the advantage of providing a recording medium which exhibits the performance equal to, or greater than, that of a coating type magnetic recording medium (a magnetic recording medium formed by applying a mixture of a magnetic powder and an organic polymer binder onto a non-magnetic support) whose magnetic recording layer has a thickness of no less than 3 .mu.m.
There is a concept that magnetic properties such as coercive force Hc, which is a static property, or squareness ratio of hysteresis loop, of the magnetic recording medium do not appreciably depend on surface conditions of the non-magnetic support used. As a product embodying such a concept, U.S. Pat. No. 3,787,327 discloses a Co-Cr multi-layered structure formed by means of vacuum evaporation.
With regard to magnetic recording media with thin metallic film, however, the metallic film formed on a non-magnetic support surface is extremely thin, and the surface condition (surface unevenness) of the non-magnetic support is faithfully reflected as the surface unevenness of the magnetic recording layer. This is considered as a defect of this type of media, because the surface unevenness gives a cause of undesirable noise.
In order to eliminate noise, it is preferred that the non-magnetic support has a surface as smooth as possible. Whereas, for handling, such as winding and unwinding, of the base film, a film with smooth surface has a poor film-to-film slip property and in consequence, a blocking phenomenon occurs and such film cannot be used as a commercially acceptable product. For this reason, the base film is required to have a rough surface. Thus, from the standpoint of electromagnetic performance, the non-magnetic support is required to have a smooth surface, while from the standpoint of handling property, the surface is required to be rough. It is therefore necessary that both of those contradictory requirements are satisfied simultaneously.
Another important problem in actual use of magnetic recording media of thin metallic film type is the running property of the thin metallic film surface. With conventional coating type magnetic recording media prepared by mixing magnetic powder with an organic polymer binder and then applying the mixture onto the base film, the running property of the magnetic surface can be improved by dispersing a lubricant in the binder. However, such a means cannot be adopted with the metallic film type magnetic recording media, and it is extremely difficult to stably maintain satisfactory running property. The media have a defect that their running property is inferior, particularly under high temperature and high humidity.
For the purpose of improving this defect, Japanese Patent Publication No. 30105/1987 proposes to form fine projections on the film surface, using fine particles, a water-soluble resin and a silane coupling agent. Japanese Patent Publication No. 30106/1987 and Japanese Laid-Open Patent Publication No. 229316/1984 also propose to form fine projections on the film surface, using fine particles and a water-soluble resin. In all of those proposals, however, fine particles are present in trapezoidal projections of a water-soluble resin, and the fine particles are not uniformly present on the film surface. Whereas, Japanese Patent Publication No. 34456/1989 proposes to intimately adhere onto the film surface a non-continuous coating film of a water-soluble polymer and fine particles forming projections higher than said coating, independently of each other. The product, however, is inferior in uniformity on the film surface, because the projections are provided by the non-continuous coating film and the particles which are not uniformly dispersed.
Furthermore, Japanese Laid-Open Patent Publication No. 53815/1987 proposes to form, on a surface of polyester film on which a thin metallic film is to be formed, projections of 0.01 to 0.1 .mu.m in diameter and 10 to 1,000 .ANG. in height, at the number of 10.sup.6 to 10.sup.9 projections/mm.sup.2 ; another Japanese Laid-Open Patent Publication No. 64545/1987 proposes to form, on a surface of a polyester film on which a thin metallic film is to be formed, projections of 0.2 to 2 .mu.m in diameter and 20 to 200 .ANG. in height at a distribution density of 10.sup.3 to 10.sup.6 projections/mm.sup.2, and those of 0.01 to 0.1 .mu.m in diameter and 10 to 1,000 .ANG. in height, at a distribution density of 10.sup.6 to 10.sup.9 projections/mm.sup.2. The polyester films described in working examples of these Japanese Laid-Open Patent Publications, however, are composed of polyesters prepared by the use of anitmony trioxide as the polymerization catalyst and have a surface defect considered to be attributable to the antimony trioxide. As an attempt to overcome such defect, the International Laid-Open Publication WO 88/08437 proposes to use a germanium compound as the polymerization catalyst, however, without yet achieving sufficient effect.